D.C. Code § 35-209: Type of rails required.

No other rail than a flat grooved rail made level with the surface of the streets upon each side of the tracks or roadbeds, so that no obstruction shall be presented to vehicles passing over said tracks, shall be laid by any street railway company in the streets of Washington; provided, that the foregoing requirements as to rails and roadbed shall not apply to street railroads outside the City of Washington.